Duplex stainless steel having excellent corrosion resistance and method for manufacturing the same

ABSTRACT

The present invention relates to a duplex stainless steel, and to a duplex stainless steel having excellent corrosion resistance and a method of manufacturing the same.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority to Korean Patent Application No. 10-2016-0177540, filed on Dec. 23, 2016 with the Korean Intellectual Property Office, the entirety of the disclosures of which is incorporated herein by reference.

BACKGROUND

The present disclosure relates to a duplex stainless steel, and particularly, to a duplex stainless steel having excellent corrosion resistance and a method of manufacturing the same.

Stainless steel is a steel material having chromium (Cr) in an amount of 18% or greater for high corrosion resistance, and is classified, according to chemical composition or metallurgical structure, as an austenitic stainless steel, a ferritic stainless steel, a precipitation hardening stainless steel, a martensitic stainless steel, or a duplex stainless steel.

A duplex stainless steel is a stainless steel having a structure in which austenite and ferrite phases are mixed. Such a duplex stainless steel has the merits of austenitic stainless steel and ferritic stainless steel as well as a high degree of strength. In detail, since a duplex stainless steel has higher strength and corrosion resistance than a stainless steel according to the related art, a range of applications such as in thermal power plant piping, a flue gas desulfurizer (FGD) duct, a seawater cooling pipe for a nuclear power plant, a chemical tank for shipbuilding, and the like, has been increased.

However, since such a duplex stainless steel contains a large amount of relatively expensive elements such as nickel (Ni), molybdenum (Mo), and the like, manufacturing costs may be increased, so there may be disadvantages in terms of price competitiveness, as compared to other grades of steel.

In this regard, recently there has been increasing interest in a cost-effective lean duplex stainless steel to which relatively inexpensive alloying elements are added, while significantly reducing the addition of relatively expensive alloying elements such as Ni, Mo, and the like (Patent Document 1). However, there is a problem that corrosion resistance may be lowered in a corrosive environment containing chlorine, due to an influence of a reduced amount of elements improving corrosion resistance, such as an alloy of Ni, Mo, and the like.

Therefore, in manufacturing such an economical duplex stainless steel, it is important to secure sufficient corrosion resistance.

RELATED ART DOCUMENT

(Patent Document 1) Korean Patent No. 10-1379079

SUMMARY

An aspect of the present disclosure provides a duplex stainless steel having improved corrosion resistance without a separate coating process, by transforming a surface structure of a duplex stainless steel, and a method of manufacturing the same.

The scope of the present disclosure is not limited to the above-mentioned aspects. Other aspects of the present disclosure are stated in the following description, and the aspects of the present disclosure will be clearly understood by those having ordinary skill in the art through the following description.

According to an aspect of the present disclosure, a duplex stainless steel having excellent corrosion resistance may be a duplex stainless steel, wherein a surface portion of the steel includes an austenite structure having an area fraction of 85% or more, and an interior of the steel includes ferrite and austenite.

According to another aspect of the present disclosure, a method of manufacturing a duplex stainless steel having excellent corrosion resistance may include: preparing a duplex stainless steel; and bright annealing heat treating the duplex stainless steel in a reducing atmosphere at a temperature within a range of 1000° C. to 1200° C. for 10 seconds or more to 30 minutes or less.

BRIEF DESCRIPTION OF DRAWINGS

The above and other aspects, features, and advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is an image of a cross section of Comparative Example 1 in an example of the present disclosure;

FIG. 2 is an image in which corrosion resistance of the duplex steel sheet of FIG. 1 is evaluated and observed;

FIGS. 3A and 3B are images of a portion in which corrosion occurs in FIG. 2;

FIG. 4 is an image of a cross section of Inventive Example 1 in an example of the present disclosure; and

FIG. 5 is an image in which corrosion resistance of the duplex steel sheet of FIG. 4 is evaluated and observed.

DETAILED DESCRIPTION

The inventors of the present disclosure have researched corrosion of a duplex stainless steel having an austenite structure having an area fraction of 40% to 60% and a ferrite structure having an area fraction of 40% to 60%, and as a result, have recognized that corrosion occurs in ferrite surrounded by austenite, as a corrosion location, so the present disclosure has been invented to address this issue.

In view of the fact that nitrogen is an austenite stabilizing element, nitrogen was adsorbed on the duplex stainless steel to induce transformation of a ferrite structure on a surface into austenite, so as to improve corrosion resistance.

The method of adsorbing nitrogen on a surface may be a nitrification treatment. Such a nitrification treatment is a method of forming a nitride on a surface of a general steel to improve corrosion resistance, abrasion resistance, fatigue strength and the like. Examples of such a nitrification treatment include plasma nitrification, gas nitrification, liquid nitrification, and the like. By these methods, nitrogen may be adsorbed on the surface of the stainless steel, but it is not advantageous in terms of productivity and material properties, so a method for addressing this problem has been devised.

Hereinafter, embodiments of the present disclosure will be described in detail. First, a duplex stainless steel of the present disclosure will be described in detail.

In the duplex stainless steel of the present disclosure, a microstructure of a surface portion preferably includes austenite having an area fraction of 85% or more, and an interior preferably includes austenite and ferrite.

The surface portion is preferably up to ⅓ of a steel thickness. For austenite transformation through adsorption on a surface, a large amount of nitrogen should be solidified in steel. In the case of an austenite phase including an excessive amount of nitrogen, brittleness may occur due to nitrogen, so unique duplex characteristics may be lost. Moreover, a considerable amount of time is required for diffusion of nitrogen beyond a ⅓ point. Thus, it is not preferable to allow the austenite phase to include an excessive amount of nitrogen.

The content of nitrogen (N) of the surface portion is preferably 0.4 wt % to 1.0 wt %. If the content of nitrogen of the surface portion is less than 0.4 wt %, a heat treatment temperature for transforming a ferrite phase in a surface into an austenite phase should be significantly high. If the content of nitrogen is increased to be greater than 1.0 wt %, a considerable amount of time should be required for nitrification.

The interior of the duplex stainless steel preferably has a duplex stainless steel structure according to the related art. For example, the interior thereof preferably includes ferrite having an area fraction of 40% to 60% and austenite having an area fraction of 40% to 60%.

An example of a composition of a duplex steel to be applied to the present disclosure is as follows. The duplex steel includes: C: 0.01 wt % to 0.10 wt %, Si: 0.5 wt % to 1.5 wt %, Mn: 1.0 wt % to 5.0 wt %, P: 0.03 wt % or less, S: 0.02 wt % or less, Cr: 19.0 wt % to 23.0 wt %, Ni: 0.5 wt % to 6.5 wt %, Mo: 3.5 wt % or less, Cu: 0.1 wt % to 1.5 wt %, N: 0.1 wt % to 0.3 wt %, and a balance of iron (Fe) and inevitable impurities. The composition of the steel refers to the interior composition of the steel. As described above, the surface portion has a slightly different content of N.

Carbon (C): 0.01% to 0.1%

Carbon (C), an austenite phase forming element, is effective in increasing the strength of a material through solid-solution strengthening. To this end, it is required to be included in an amount of 0.01% or more. If the content of carbon (C) is greater than 0.10%, carbide-forming elements such as chromium (Cr), effective in improving corrosion resistance, may easily combine with carbon (C) along austenite-ferrite boundaries, and thus, the content of chromium (Cr) may be decreased along grain boundaries to cause a decrease in corrosion resistance. Therefore, preferably, the content of carbon (C) may be 0.1% or less.

Silicon (Si): 0.5% to 1.5%

Silicon (Si) is added to obtain a deoxidizing effect to some degree and acts as a ferrite phase forming element which concentrates in ferrite during an annealing process. Therefore, silicon (Si) may be added in an amount of 0.5% or greater to obtain a proper ferrite phase faction. However, if the content of silicon (Si) is greater than 1.5%, hardness of a ferrite phase sharply increases to cause a decrease in elongation, and thus, it may be difficult to form an austenite phase having an effect of guaranteeing elongation. In addition, if silicon (Si) is added excessively, the fluidity of slag is low in a steel making process, and since silicon (Si) forms inclusions by combining with oxygen, corrosion resistance decreases. Therefore, preferably, the content of silicon (Si) may be adjusted to be within the range of 0.5% to 1.5%.

Manganese (Mn): 1.0% to 5.0%

Manganese (Mn) is an element increasing the amount of a deoxidizer and the solid solubility of nitrogen (N), and is added as an austenite-forming element to replace relatively expensive nickel (Ni). If the content of manganese (Mn) is greater than 5.0%, it is difficult to obtain corrosion resistance as high as that of 304 steel, and manganese (Mn) combines with sulfur (S) in steel to form MnS which decreases corrosion resistance. Conversely, if the content of manganese (Mn) is less than 1.0%, it is difficult to guarantee a proper austenite phase fraction even though austenite-forming elements such as nickel (Ni), copper (Cu), or nitrogen (N) are adjusted, and the solid solubility of nitrogen (N) to be added is too low to sufficiently dissolve nitrogen (N) at atmospheric pressure. Therefore, it may be preferable that the content of manganese (Mn) be within the range of 1.0% to 5.0%.

Chromium (Cr): 19.0% to 23.0%

Chromium (Cr), being an element stabilizing ferrite, together with silicon (Si), guarantees the corrosion resistance of a duplex stainless steel in addition to playing a major role in forming a ferrite phase in a duplex stainless steel. If the content of chromium (Cr) increases, corrosion resistance increases. In this case, however, it is necessary to increase the content of relatively expensive nickel (Ni) or the contents of other austenite-forming elements to maintain phase fractions. Therefore, preferably, the content of chromium (Cr) may be adjusted to be within the range of 19.0% to 23.0% to obtain corrosion resistance equal to or higher than that of 304 steel while maintaining phase fractions of a duplex stainless steel.

Nickel (Ni): 0.5% to 6.5%

Nickel (Ni) functions as an austenite-stabilizing element together with manganese (Mn), copper (Cu), and nitrogen (N) and plays a major role in guaranteeing the formation of an austenite phase in a duplex stainless steel. The content of relatively expensive nickel (Ni) may be maximally reduced for cost reductions, and in this case, the contents of manganese (Mn) and nitrogen (N) having a function of forming an austenite phase may be increased to maintain balance between phase fractions in spite of a decrease in the content of nickel (Ni). However, the content of nickel (Ni) may be adjusted to be 0.5% or greater so as to suppress the formation of strain-induced martensite, generated during cold working, and to sufficiently guarantee the stability of an austenite phase. If nickel (Ni) is added in large amounts, it is difficult to maintain a proper austenite fraction because the fraction of an austenite phase increases, and particularly, production costs of a product increase due to relatively expensive nickel (Ni), making it difficult to guarantee the competitiveness to 304 steel. Therefore, preferably, the content of nickel (Ni) may be within the range of 0.5% to 6.5%.

Nitrogen (N): 0.1% to 0.3%

Together with nickel (Ni), nitrogen (N) significantly contributes to stabilizing an austenite phase in a duplex stainless steel, and is an element concentrated in an austenite phase during an annealing heat treatment process. Therefore, if the content of nitrogen (N) is increased, corrosion resistance and strength may be concomitantly improved. However, since the solid solubility of nitrogen (N) may vary according to the content of manganese (Mn), it is necessary to adjust the content of nitrogen (N). If the content of nitrogen (N) is greater than 0.3% when the content of manganese (Mn) is within the range proposed in the present disclosure, the content of nitrogen (N) exceeds the solid solubility of nitrogen (N), and thus, surface defects may be caused because of the formation of blow holes and pin holes during a casting process. Nitrogen (N) may be added in an amount of 0.1% or greater to obtain corrosion resistance as high as that of 304 steel, and if the content of nitrogen (N) is excessively low, it is difficult to maintain proper phase fractions. Therefore, it may be preferable that the content of nitrogen (N) be within the range of 0.1% to 0.3%.

Molybdenum (Mo): 3.5% or less (excluding 0%)

Molybdenum (Mo) is a stronger element improving corrosion resistance, as compared to chromium (Cr). A higher Mo content is advantageous, taking into account a pitting corrosion resistance equivalent index. However, when a large amount of Molybdenum (Mo), as a ferrite stabilizing element, is added, it is difficult to obtain a duplex structure. Moreover, since it is difficult to be transformed into austenite during nitrification treatment, the content of Molybdenum (Mo) is preferably 3.5% or less.

Copper (Cu): 0.1% to 1.5%

Copper (Cu) is an austenite stabilizing element. When an appropriate amount of copper (Cu) is added, elongation of a duplex stainless steel is increased, and corrosion resistance in a sulfuric acid atmosphere may be improved. However, when an excessive amount of copper (Cu) is added, copper (Cu) may not be solidified in a matrix, so a problem in which a pitting potential falls in an atmosphere containing chlorine may occur. Thus, the content of copper (Cu) is preferably 0.1% to 1.5%.

Meanwhile, other than the composition described above, phosphorous (P) and sulfur (S) may be included. Phosphorous (P) and sulfur (S) are elements which easily segregate in grain boundaries during solidification. When phosphorous (P) and sulfur (S) segregate during solidification, a low melting point phase is formed, so hot workability may be reduced and thus cracking may occur. Therefore, the contents of the phosphorous (P) and sulfur (S) are preferably adjusted to be as low as possible. In this regard, in the present disclosure, the content of phosphorous (P) is 0.03% or less, and the content of sulfur (S) is 0.02% or less.

The other component of the present disclosure is iron (Fe). However, unintended impurities of raw materials or manufacturing environments may inevitably be included in a manufacturing process according to the related art, and thus cannot be excluded. Such impurities are well-known to those of ordinary skill in manufacturing industries, and thus, specific descriptions of the impurities will not be given in the present disclosure.

Hereinafter, a method of manufacturing a duplex stainless steel of the present disclosure will be described in detail.

First, a duplex stainless steel having the composition described above may be prepared. A method of manufacturing the duplex stainless steel is not particularly limited, and may be sufficient as long as a person skilled in the art can understand the present disclosure. As an example, the duplex stainless steel may be manufactured using a twin roll strip casting method.

The duplex stainless steel may be obtained by performing cold rolling for a desired thickness, after a hot rolled steel sheet is manufactured.

With respect to the duplex stainless steel, prepared as described above, a bright annealing heat treatment may be performed. The bright annealing heat treatment is preferably performed under conditions in which a dew point of an atmosphere gas is −20° C. to −80° C. During the bright annealing heat treatment, when a dew point of the atmosphere gas increases, due to a reaction of oxygen and a steel sheet, a Cr oxide film becomes dense on a surface. Thus, a movement path of nitrogen, diffused to a surface of a steel sheet in the atmosphere, is blocked, so an effective nitrification treatment may not occur. For an effective nitrification treatment, a dew point temperature of the atmosphere gas is preferably −20° C. to −80° C.

The atmosphere gas of the bright annealing heat treatment is preferably a reducing atmosphere. In addition, the bright annealing heat treatment is performed in a nitrogen (N₂) atmosphere, and is preferably performed in an atmosphere in which a volume ratio of hydrogen (H₂) to nitrogen (N₂) is 3:1 or more. In general, when ammonia (NH₃) gas is heated at 500° C. or more, a nitrogen atom, generated by pyrolysis due to iron surface catalysis, is adsorbed on a surface of a steel sheet, so the nitrogen atom is diffused into an interior of the steel sheet and thus a nitride layer is formed. In consideration of this, a ratio of hydrogen to nitrogen is preferably 3:1 or more.

A temperature of the heat treatment is preferably 1000° C. to 1200° C. In order to manufacture a thin plate, hot rolling and cold rolling are performed. In addition, annealing of a sheet having been cold rolled is performed at 1000° C. to 1200° C., for the purpose of easy transformation of an austenite phase during nitrification treatment.

The heat treatment time is preferably longer than 10 seconds and shorter than 30 minutes in a heat treatment temperature. Considering the heat treatment temperature, when the heat treatment time is 10 seconds or less, sufficient absorption is difficult to occur. When the heat treatment time exceeds 30 minutes, nitrogen penetrates into an interior of a steel sheet, so there is a risk of losing characteristics of a duplex stainless steel.

In the present disclosure, through the operation described above, a concentration of nitrogen in a surface portion of a duplex stainless steel is increased, so a surface is only changed into austenite. Thus, without additional processing and cost increases, a duplex stainless steel with improved corrosion resistance may be manufactured.

Hereinafter, the present disclosure will be described more specifically through examples. However, the following examples should be considered in a descriptive sense only, rather than for the purposes of limitation of the scope of the present disclosure.

Example

After a duplex stainless steel hot rolled steel sheet having a composition of Table 1 (wt %, with a balance of iron (Fe) and inevitable impurities) was manufactured, cold rolling was performed thereon, and a cold rolled steel sheet was manufactured. Thereafter, a bright annealing heat treatment was performed under conditions of Table 2. In this case, as an atmosphere gas, a gas in which a ratio of H₂/N₂ is 3:1 was used.

TABLE 1 Class- ification C Si Mn P S Cr Ni Mo Cu N Composition 0.04 0.72 2.96 0.0217 0.0024 20.33 0.96 0.01 0.845 0.232

Meanwhile, a nitrogen content and a pitting corrosion resistance index in a surface portion and an interior of the duplex stainless steel having been heat treated were confirmed, an austenite fraction (area %) of the surface portion was observed, and results thereof are illustrated in Table 2.

In addition, corrosion resistance of each specimen was evaluated, and results thereof are illustrated in Table 2. To evaluate corrosion resistance, an accelerated spray test based on ISO 14993 was conducted 60 times. Regarding conditions of a single test based on ISO 14993, a 5% NaCl salt water test solution was prepared and operations were performed as follows.

[Salt water spraying: 35±1° C., 2 hours]→[Drying: 60±1° C., relative humidity of less than 30%, 4 hours]→[Wetting: 50±1° C., relative humidity of more than 95%, 2 hours]

As a result of the test described above, a corrosion resistance evaluation standard is illustrated as ‘o’ when a smaller amount of rust was present than an amount of rust on a surface of STS 316, and is illustrated as ‘x’ when a larger amount of rust was present than an amount of rust on a surface of STS 316.

TABLE 2 Pitting Nitrogen corrosion Surface Heat concentration resistance portion Dew treatment Heat (wt %) index austenite Class- point temperature treatment Surface Surface fraction Corrosion ification (° C.) (° C.) time portion Interior portion Interior (area %) resistance Comparative −40 to 1050 10 0.23 0.23 24.1 24.1 50 x Example −60 seconds 1 Inventive −65 to 1050 20 0.8 0.23 33.2 24.1 90 ∘ Example −80 seconds 1 Inventive −65 to 1100 30 1 0.23 36.4 24.1 95 or ∘ Example −80 seconds more 2 Inventive −20 to 1050 30 0.45 0.23 27.6 24.1 85 ∘ Example −40 seconds 3 Inventive −20 to 1100 30 0.55 0.23 29.2 24.1 90 ∘ Example −40 seconds 4 Comparative −65 to 1100 10 0.3 0.23 25.2 24.1 60 x Example −80 seconds 2

Here, a pitting corrosion resistance index (PREN)=Cr+3.3(Mo+0.5 W)+16N, and each component numeral is the content thereof (wt %).

As illustrated in the results of Table 2, in the case of Inventive Example satisfying conditions of the present disclosure, a fraction of ferrite in a surface portion is reduced, sufficient austenite is generated, and a pitting corrosion resistance index is increased. Thus, excellent corrosion resistance may be secured. However, in the case of Comparative Examples outside of the conditions of the present disclosure, sufficient austenite may not be formed in a surface portion, so it is confirmed that corrosion resistance is inferior.

In detail, FIG. 1 is an image of a cross section of Comparative Example 1, and it is confirmed that a large amount of a ferrite structure is included in a surface portion. In FIG. 1, a bright portion represents an austenite structure and a dark portion represents a ferrite structure. After a corrosion resistance evaluation of the duplex stainless steel of FIG. 1 as described above was conducted, as a result of FIGS. 2, 3A, and 3B, it was confirmed that corrosion occurred. In detail, it was confirmed that a corrosion pit was observed in a ferrite structure surrounded by an austenite structure.

Meanwhile, in FIGS. 4 and 5 illustrating Inventive Example 1, it is confirmed that there was almost no ferrite in a surface portion, in a cross section of a duplex stainless steel. As a result, it is confirmed that corrosion resistance was excellent.

As set forth above, according to an exemplary embodiment, a structure of a surface portion of a duplex stainless steel is transformed into austenite, so a duplex stainless steel with significantly improved corrosion resistance may be provided.

While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims. 

What is claimed is:
 1. A duplex stainless steel having excellent corrosion resistance, wherein a surface portion of the steel includes an austenite structure having an area fraction of 85% or more, and an interior of the steel includes ferrite and austenite.
 2. The duplex stainless steel of claim 1, wherein the surface portion is up to ⅓ of a steel thickness.
 3. The duplex stainless steel of claim 1, wherein the content of nitrogen, included in the surface portion, is 0.4 wt % to 1.0 wt %.
 4. The duplex stainless steel of claim 1, wherein the interior of the steel includes an austenite structure having an area fraction of 40% to 60% and a ferrite structure having an area fraction of 40% to 60%.
 5. The duplex stainless steel of claim 1, wherein a composition of the steel includes carbon (C): 0.01 wt % to 0.10 wt %, silicon (Si): 0.5 wt % to 1.5 wt %, manganese (Mn): 1.0 wt % to 5.0 wt %, phosphorus (P): 0.03 wt % or less, sulfur (S): 0.02% wt % or less, chromium (Cr): 19.0 wt % to 23.0 wt %, nickel (Ni): 0.5 wt % to 6.5 wt %, molybdenum (Mo): 3.5 wt % or less, copper (Cu): 0.1 wt % to 1.5 wt %, nitrogen (N): 0.1 wt % to 0.3 wt %, and a balance of iron (Fe) and inevitable impurities.
 6. A method of manufacturing a duplex stainless steel having excellent corrosion resistance, comprising: preparing a duplex stainless steel; and bright annealing heat treating the duplex stainless steel in a reducing atmosphere at a temperature within a range of 1000° C. to 1200° C. for 10 seconds or more to 30 minutes or less.
 7. The method of claim 6, wherein a dew point temperature of the reducing atmosphere is −20° C. to −80° C.
 8. The method of claim 6, wherein, in an atmosphere gas of the reducing atmosphere, a ratio of nitrogen (N₂) to hydrogen (H₂) is 1:3 or more.
 9. The method of claim 8, wherein the atmosphere gas is 100% nitrogen (N₂). 